1. Field of the Invention
This invention relates to a laser device, and more specifically to a distributed feedback type laser device having a periodic structure for distributively feeding back a laser light.
2. Description of the Related Art
A distributed feedback type semiconductor laser device oscillating in a longitudinal single mode is a key device which is indispensable for a large-capacity and high-speed optical communication system. The distributed feedback type semiconductor laser device oscillates in two longitudinal modes in principle. For example, structures (a) and (b) described below are known in the art in order to attain the longitudinal single mode,
(a) In part of the periodic structure (diffraction grating), a portion in which the period of the diffraction grating is deviated by .pi. in phase is formed. PA1 (b) A waveguide having two or more portions having different propagation constants is arranged and a structure in which a phase of light is shifted by an integer multiple of approx. .pi./2 is formed.
The semiconductor laser device with the above construction is called a phase-shifted distributed feedback laser device. For example, such a phase-shifted distributed feedback laser device is disclosed in ELECTRONICS LETTERS, 19th Jan. 1984, Vol. 20, No. 2, pp 80 and 81, "1.5 .mu.m PHASE-SHIFTED DFB LASERS FOR SINGLE-MODE OPERATION", ELECTRONICS LETTERS, 19th Jan. 1984, Vol. 20, No. 2, pp 82 to 84, "PROPOSAL OF A DISTRIBUTED FEEDBACK LASER WITH NONUNIFORM STRIPE WIDTH FOR COMPLETE SINGLE-MODE OSCILLATION" and the like.
However, in a distributed feedback type semiconductor laser device, the photon density in the cavity sometimes markedly varies in the axial direction as shown in FIG. 1. FIG. 1 is a distribution diagram showing a relation between the axial direction of laser cavity and the photon density. As is clearly seen from FIG. 1, the photon density rapidly increases in a portion in which a phase shifter is formed. Further, the distribution of the photon density varies in the axial direction by the hole burning effect in the axial direction after the laser oscillation. As a result, there may occur a problem that the linearity of the current-light output characteristic (which is hereinafter referred to as the I-L characteristic) is deteriorated and the gain difference between the main oscillation mode and sub-oscillation mode is lowered.
In FIG. 2, the I-L characteristic and dL/dI characteristic of the conventional laser element are shown. As shown in FIG. 2, the linearity of the I-L characteristic is insufficient since the hole burning effect occurs after the laser oscillation. Further, since variation occurs in the phase of the end facet and the shape of the inherent waveguide of each laser element, the frequency of the hole burning phenomena will be changed. The above-described causes lower the manufacturing yield of distributed feedback type semiconductor laser devices. In order to suppress the occurrence of the hole burning, the standardized coupling coefficient kL may be set to an optimum value, but in practice, it is extremely difficult to control kL.